Method of making metal fibers

ABSTRACT

HARD METAL POWDER COMPACTS ARE SINTERED AND IMPREGNATED WITH A SOFTER METAL. THE COMPACTS ARE REDUCED TO ROD, WIRE OR SHEET. IN THE PROCESS FINE FIBERS OF THE HARD METAL POWDER ARE FORMED.

Aug. 1, 19

R. W. DOUGLASS METHOD OF MAKING METAL FIBERS Original Filed Marsh 29, 1967 POWDERS OF +FIRST METAL I VACUUM IMPREGNATE WITH SECOND METAL 2 Sheets-Sheet l MOLTEN SECOND METAL FIRST METAL FORMS FIBERS} ELONGATE To ROD A I I DRAW TO C Ro LL TO B wIRE SHEET I C\ T n L DIFFUSION D REACTION 2) USE AS I (H coMRosITE SECOND METAL (0) USE As USE AS c x METAL FELT COMPOSITE A R |MPREGNATE- I I (b) SEPARATE DIFFUSION FIBERS REACTION FEG.E

1972 R. w. DOUGLASS 3 681,063

METHOD OF MAKING METAL FIBERS Original Filed March 29, 1967 2 Sheets-Sheet 2 3,681,063 Patented Aug. 1, 1972 U.S. Cl. 75-208 4 Claims ABSTRACT OF THE DISCLOSURE Hard metal powder compacts are sintered and impregnated with a softer metal. The compacts are reduced to rod, wire or sheet. In the process fine fibers of the hard metal powder are formed.

This application is a division of my copending application Ser. No. 626,773, filed Mar. 29, 1967, now abandoned.

The present invention relates to metal fibers or filaments useful for a variety of purposes including capacitors, filters, structural reinforcement. The fibers are particularly of the class of hard metals having high strength and high temperature use capability (having at least 50% room temperature strength at 500 C.) and extraordinarily small diameter as on the order of a micron or less, while having continuous length of several times diameter and as high as ten inches.

The invention relates to such filaments as separate entities, in loose bundles (i.e. a metal felt) or as incorporated in reinforced matrices and to the process of making them.

BACKGROUND Metal felts and fine metal wires or fibers used in such felts are known in the art as indicated in Pats. 2,903,787 and 3,178,280. These felts are made from standard cold reduced metal wires which are limited to minimum diameters on the order of .001.010 or less by the inherent vulnerabilities of standard wire drawing processes or from shavings from metal blocks which are characterized by many surface defects. Much finer wires can be made by extrusion as indicated in Pat. 3,199,331 to Allen. But production by this process is substantially limited as a practical matter to low melting metals and alloys (e.g. tin). Other prior art of interest is Buchler, US. Pat. 3,124,- 455 and the Speidel, Levy and Wulfi work cited below.

The present invention involves as a principal object the production of metal fibers of sub-micron size by a new process which is capable of being used with high temperature metals such as tantalum.

It is a further object of the invention to provide an economical method of making metal fibers on the order of microns or less, and preferably sub-micron, in diameter with a single series of processing steps; i.e. free of the expensive supplementary or recycling processing involved, for instance, in Speidel, US. Pat. 3,256,118, Levy, US. Pat. 3,029,496 and Wulff, January 1966 Journal of Applied Physics, p. 5.

It is a further object of the invention to provide work hardened fibers by a production process free of the need for intermediate anneals as required in the above patents of Allen and Levy, and for use in composites providing a high degree of work hardening in final product form, with or without a final low anneal for stress relief of the matrix only.

Other objects, features and advantages of the present invention will in part be obvious and will in part appear hereinafter.

DESCRIPTION The invention is now described with respect to typical specific embodiments thereof and with reference to the accompanying drawings wherein:

FIG. 1 is a block diagram of the process of the invention.

FIG. 2 is a copy of a photomicrograph of a composite according to the invention.

FIG. 3 is a copy of a photomicrograph of a metal felt according to the invention.

The fibers of the invention are made and used by the following process described with reference to FIG. 1 which is a block diagram of the process. First, powders of the metal to be fibered are obtained. The metal may be any of tantalum, niobium, molybdenum, tungsten, iron or stainless steels, titanium, nickel, aluminum, chromium, beryllium, magnesium oxide, titanium hydride and fabricable aluminides and silicides or other hard metal elements, compounds or alloys which have softening temperatures in excess of about 1000 C. The starting powder size is variable depending upon subsequent processing and reactivity of the powders. The invention has been practiced successfully for instance with tantalum powders as large as minus mesh and as small as a few microns diameter. The powder is consolidated into a compact by pressing and sintering or sintering in a mold. Then a melt of a second metal is provided in vacuum or inert atmosphere and the powder compact of the first metal is impregnated by dipping in the melt. During both the sintering and impregnating steps the compact is degassed and purified to enhance its wettability and ductility.

The second metal may be any of aluminum, copper, nickel, Woods metal, tin, indium, mercury, or any other metal which meets the following criteria with respect to the first metal under the conditions of impregnation:

(l) readily wet the skeleton structure of the sintered compact of the first metal.

(2) not alloy extensively with the first metal.

(3) have similar hardness and fabrication characteristics to the extent necessary for co-working.

(4) be easily removable from the compact by chemical or thermal means.

The impregnated compact is then worked down to an elongated rod form or the like e.g. plate or cylinder (round or rectangular cross section) by swaging or forging. During this process the adjacent particles of hard metal in the compact begin to form long fibers within the matrix of the second metal.

At this point, the rod or cylinder or plate may be used or fabricated into a useful product in any of the following ways:

A( 1) Removing the matrix metal and (a) using directly as a filter or with further fabrication as a capacitor (b) separating out individual fibers (c) re-impregnating the fibered article A-(2) Using the rod directly as a composite structural element B Rolling the rod to sheet prior to (l) or (2) above C Drawing the rod to wire prior to (1) or (2) above D Heating the rod for diffusion reaction between the hard metal fibers and the matrix metal prior to 1) or (2) above.

Several permutations of the foregoing can be made. For instance a rod can be drawn for several passes before rolling. A wire or sheet can be heated for diffusion reaction. Similarly a re-impregnated article can be used as a composite, with or without a diffusion reaction, or releached. With difiusion reactions, fibers of alloys or compounds can be formed even though such alloys are too brittle to be fibered directly. Another alternative in the scope of the invention is to form a loose fiber bundle or separate fiber (a or b above) and expose it to an oxidizing or nitriding atmosphere. In this way fibers of aluminum oxide or aluminum nitride can be made for use in reinforced composite structures. Also fibers of tantalum or niobium nitride can be made for use as superconductors. In these applications it is of special interest that the fiber diameters are so small as to favor the formation of the above compounds in single crystal form which is especially desirable.

The fibers of the invention are characterized in that each fiber is derived from a single powder particle and its length is dependent on the degree of diameter reduction. For instance, an 8 micron diameter powder particle fibered to 0.1 micron diameter will have a length of about one inch, a 30 micron diameter particle fibered to 0.1 micron diameter will have a length of about seventy inches. Further cold working to finer fiber diameters would increase the length. In most applications of the invention, useful fibers will have a length of ten times the diameter of the fiber or longer (as high as 10 times for extreme cases).

The felts of the invention are characterized by substantial cross-linking by metallurgical bonds between tangentially contacting fibers corresponding in part to the bonds between powders in the original powder compact skeleton and corresponding in part to new bonds formed during cold working the impregnated compact down to an elongated article.

FIG. 2 shows longitudinal section photomicrograph of a composite in the form of a wire of 0.39 inch diameter at 133 times magnification. The composite has elongated reinforcing tantalum fibers in a matrix of copper. The starting material for the fibered metal was coarse melting grade powder minus 12 and plus 60 mesh pressed at 18,000 p.s.i. and sintered at 2300 C. for one hour to produce a compact of 61% density.

FIG. 3 shows a longitudinal section photomicrograph of a tantalum metal felt, encapsulated in a molding resin for microscope examination, at 266 times magnification. The tantalum was made from nominal 8 micron diameter powders (minus 100 mesh and plus 5 microns) which was consolidated to a compact of about 50% density and then impregnated with copper and then swaged to rod and rolled to sheet after which the copper was leached out in a nitric acid bath. Upon leaching the metal felt ballooned up to several times its original volume.

Fibers obtained from rod or wire are found to be essentially circular in cross-section and fibers obtained from sheet are found to be rectangular in cross-section. The term diameter as used herein refers to diameter of a circle or width of a rectangle.

The practice of the invention is further illustrated by the following non-limiting examples.

EXAMPLE 1 A mold was filled with tantalum powder of about 8 micron nominal diameter (-100 mesh and plus 5 microns) and the powder was sintered in the mold at 1500 C. for 20 minutes to form a green compact. Then sintering was completed by removing the compact from the mold and heating at 2300 C. for one hour to complete consolidation of the powder. The density of the compact was 8.22 gms./cc. or 49.5% of theoretical density. The compact was vacuum impregnated with copper by dipping in a molten copper bath at 1170 C. for 5 minutes under a vacuum of about 10* torr. The impregnated compact (.35 inch diameter by 4 inches long) was enclosed in an iron pipe and then swaged to .125 inch diam eter. The jacket was removed and the rod was then further swaged to .080 inch diameter. After swaging the rod was then leached in nitric acid to remove the copper. The leached compact left a bundle of interwoven tantalum fibers in the form of a felt.

This metal felt was rinsed and removed from the leach bath. The felt was anodized and formed into a capacitor anode and tested for capacitor properties in a wet electrolyte. The formation voltage was 200 volts and the capacitance was 30.6 microfarads and on a specific weight basis 6120 microfarad-volts per gram. The felt bad a dissipation factor of 32.19% making it an over-all operable capacitor anode.

EXAMPLE 2 Tantalum felts were made as in Example 1 but with the difference that the compact was rolled to .010 inch thick sheet before leaching. The felt exhibited a vigorous swelling with a volume increase and density decrease of 510 times up during leaching and floated on the leaching bath. A capacitor formed from the felt at volts had 7965 microfarad-volts per gram specific capacitance.

EXAMPLE 3' Felts were made as in Example 1 and 2 with the difference that consolidation of the tantalum powder was accomplished by pressing at 18,000 p.s.i. and then sintering at 2250 C. for one hour and that some rods were drawn to Wire. Densities of 6080% of theoretical were obtained in the original compact. Upon leaching the final composite articles of this type, the felt did not swell up. However, high values of capacitance were still obtained indicating substantial formation of new surface as in Examples 1 and 2 (surface enhancement of about 2.5 times).

EXAMPLE 4 Several fibers from the felts of Examples 1 and 2 were encapsulated in epoxy resin and measured to yield an individual fiber diameter indication of .0002 cm. diameter. The Example 2 fibers were 5 to 10 times as long as the diameter of the fiber; the Example 1 fibers were continuous over much longer lengths.

EXAMPLE 5 TABLE 1 Ultimate tensile Example 5 sample: strength, p.s.i.

(a) .01-.020 inch diameter wire as worked (b) wire with stress relief (0) sheet, as worked ((1) sheet, stress relieved 93,000 (e) Pure tantalum, as worked (.005 and .015 inch thick sheet) 104,000-116,000 (f) Pure copper, as worked (.005

and .015 inch thick sheet) 59,000-60,500

EXAMPLE 6 A molybdenum-copper composite was made and tested in the same manner as the tantalum-copper composites of Example 5 and formed into .06 and .08 in. wire which displayed ultimate tensile strengths of 81,700 and 108,000 p.s.i., respectively.

EXAMPLE 7 Tantalum felts made as in Examples 4 and 2 were tested for tensile strength after leaching out the copper. The results are in Table 2.

TABLE 2 Example sample: Ultimate tensile strength, p.s.i. (a) .01 in. sheet 114,700 (b) .04 in. wire 90,000

EXAMPLE 8 Iron powder of 270 mesh was mold sintered at 800 C. for 20 minutes and then finally sintered at 1150 C. for 1 hour to a density of 3.45 grams per cc. (45% theoretical) impregnated as above and worked to .025 inch wire and leached to form a fibrous bundle of iron fibers, .0015 cm. diameter, quite continuous and having a surface layer of copper-iron alloy overlaid by residual copper but with a substantial core of pure iron in the fibers.

EXAMPLE 9 Before leaching, the iron-copper composite wire of Example 8 was tested for tensile strength and this was found to be 160,000 p.s.i.

EXAMPLE 10 Leaching experiments were conducted and a solution of five parts ammonium hydroxide in one part hydrogen peroxide was found to be superior to nitric acid for selectively leaching copper from the iron to free the iron fibers from the composites.

The best mode of using the invention is believed to be selection of a tantalum-copper pair to produce a tantalum felt suitable for use as a capacitor anode. In addition to the above indicated advantages of ease of processing, surface enhancement and work hardening it is a further useful advantage of the invention that it may be practiced if desired, with relatively coarse melting grade tantalum powder in the original compact rather than the conventional fine grain capacitor grade powder and the desired surface area increase can be obtained in the fibering process rather than in the original processing of the powder. A further useful aspect of the invention is the above described feature of swelling when the original compact is made in low density (40-60% theoretical) and/or when a high degree of working is put into the composite. The swelling of the metal felt, when utilized makes it easier to refill the felt with an anodizing medium and electrolyte.

The extension to other species of the above advantages and variations in processing and still other advantages and variations will be obvious to those skilled in the art from the description herein. For instance, a niobium-tin pair could be utilized to obtain interconnected niobium fibers in a tin matrix with a better degree of interconnection between fibers than is obtainable in the process of the the above described Speidel patent. Then the composite could be heated for diffusion reaction to form a niobium stannide superconductor subsequent to which residual tin would be leached out and replaced with copper by reimpregnation to provide a higher conductivity matrix for electrical stability of the superconductor.

A high degree of control of the final product is obtainable. For instance, use of coarse melt grade powders or low density consolidation of the original compact (40- 60%) tend to limit the number of cross-link bonds formed between fibers thereby enhancing the swelling up of fibers upon leaching the matrix metal and enhancing the ease of separation of fibers.

For superconductor applications it is particularly desirable to use a fine grain powder and form the original compact to a higher density for forming maximum crosslinks between fibers.

Still other applications within the scope of the present invention will be apparent to those skilled in the art when aided by the foregoing description. The description is therefore intended to be read as illustrative and not in a limiting sense.

What is claimed is:

1. Method of making a bundle of fibers comprising the 0 steps of:

(a) forming a first powdered metal material into a porous compact with metallurgical bonds between constituent particles of the material,

(b) impregnating the compact with a second metal in fluid form, the second metal when in solid form having hardness and fabrication characteristics similar to the first material to fiber the two materials when they are co-worked,

(c) solidifying the impregnant,

(d) mechanically working the impregnated compact down to an elongated form by the application of compressive forces so that the first material is fibcred into an elongated bundle of fibers with spaced cross links of elongated metallurgical bonds between the fibers corresponding to original compact bonds,

whereby a final composite product having a matrix of the second metal reinforced by an internal metal felt of worked fibers of the first material is obtained.

2. The method of claim 1 comprising the further step of (e) removing one of said first or second metals from the final composite product to leave a felt metal.

3. The method of claim 2 wherein the felt is (f) reimpregnated with a new filler.

4. The method of claim 2 wherein the second metal is the one removed.

References Cited UNITED STATES PATENTS JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R.

-DIG. 1, 200, 222', 20456 

